It is known to electronically amplify mechanically produced sounds. It also is known to produce new sounds or tones with a new timbre from mechanically produced sounds by mixing in electronically produced, harmonic tones, such as is e.g. described in DE-OS 26 56 298. However, this mixing is always associated with a time lag, because the electronic system must analyze the mechanically produced tones and must synthesize the additional tones.
The known, electronic harmonic generators and the like can simultaneously produce additional overtones and undertones, but in the case of a played chord, the whole chord is processed as a sound.
It would be desirable for each individual tone of a played chord to simultaneously extend the sound impression by additional, harmonic undertones and/or overtones. As a result sounds could be produced, which could not be produced either purely electronically, or purely mechanically. In particular, such a system would be desirable for a solid-body guitar, whose sound can decay over a very long period as a result of the complete omission of a sound board, but loses fullness as a result of the lack of the "tones" of this sound board.
Due to the lack of resonances and interferences and in particular tones, which are lower than the played fundamental tone, the amplified guitar has a downwardly limited sound impression. Up to now the undertones and overtones have been created by monophonic electronic limitation. However this leads to aggressively sounding, unharmonic interferences and playing is restricted to simple harmonic intervals. The electronic limitation of the sounds of individual strings avoids unharmonic interferences, but only creates overtones and no undertones.